Liquid crystal polymers

ABSTRACT

A liquid crystal polymer or copolymer is disclosed of structure (A) where X=CN, COR, CO 2  R, CHO, CF 3  ; R=branched chain, straight chain or chiral alkyl; ▭=any suitable mesogenic group; Z=single covalent bond, oxygen, sulfur, CO 2  or OCO; n=1 to 20; Y=H, OH, F, OCH 3  ; m=3 to 200; P=a single covalent bond, oxygen, or OCO. ##STR1##

This is a 35 U.S.C. § 371 national phase application of PCT/GB94/00662filed Mar. 30, 1994.

This invention concerns novel liquid crystal polymer (LCP) materials,novel intermediates and method for preparing same.

BACKGROUND OF THE INVENTION

LCP's are known and are used in the electro-optical device industry, forexample, in optical storage devices, in non-linear optical devices andin pyroelectric devices, see for example, GB 2146787 A and MakromolChem, 186 2639-2647, 1985.

One known type of LCP consists of a polymeric backbone to which areattached laterally pendant side chains which have a chemical structurethat is mesogenic, ie. that induces liquid crystalline character, thesebeing known as side chain liquid crystal polymers. Work in this fieldhas identified a large number of side chain structures which aresuitable, see for example GB 2146787 A. For some purposes it isdesirable that the LCP shows a smectic C (S_(c)) or chiral smectic C(S_(c).sup.•) liquid crystal phase, and a particularly preferred sidechain for achieving this is one which contains a laterally fluorinatedbiphenyl or terphenyl system.

Side chain liquid crystal polyacrylates and polymethacrylates are known,Polymer Communications, 24 364-365, 1988; Liquid Crystals 12(2) 305-318,1992, having a polyacrylate or polymethacrylate backbone, with dependentmesogenic side chains, ie. of general structure: ##STR2##

Where (CH₂)_(m) is the spacer group, X is the mesogenic side chain and Ris hydrogen in the case of polyalkylacrylates, being methyl inpolymethacrylates.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a synthetic scheme for the preparation of liquid crystalpolymers;

FIG. 2 illustrates a liquid crystal device; and

FIG. 3 illustrates a pyroelectric device.

SUMMARY OF THE INVENTION

According to the present invention there is provided a novel LCPmaterial having improved liquid crystal properties over known LCPmaterials and which my comprise the same or different monomer units, ie.may comprise polymers or copolymers, and having a general structure (A):##STR3## wherein X=CN, COR, CO₂ R, CHO, CF₃ ;

R=branched chain, straight chain or chiral alkyl;

▭=any suitable mesogenic group;

Z=single covalent bond, oxygen, sulphur, CO₂ or OCO;

n=1 to 20; preferably n=2 to 12 and more preferably n=5 to 12;

Y=H, OH, F, OCH₃ ;

m=3 to 200; preferably m=3 to 100;

P=a single covalent bond, oxygen, or OCO.

The mesogenic group my be defined from structure (B); ##STR4## whereinA, B and D are independently selected from: ##STR5## W₁ and W₂ areindependently selected from: a single covalent bond, COO, OCO, CH₂ CH₂,CH₂ O, OCH₂, 0;

Q is selected from: CN, halogen, R, OR, COOR, CF₃, lactate derivatives,where R=chiral, straight or branched chain alkyl and includes where oneor more CH₂ groups can be substituted by CH(CN), CH(CF₃), CH(Cl),CH(CH₃), where if more than one CH₂ group is substituted then suchsubstitution is not carried out on neighbouring CH₂ groups.

The copolymer my be random, block or any other variation as is known inthe art.

Preferably Y may be OH and the OH groups used as a point of attachmentfor cross-linking agents to form elastomers, examples of suchcross-linkage agents include. ##STR6##

According to another aspect of the present invention there are providedintermediate compounds, or monomer units, which, although not themselvesliquid crystalline, polymerise to form LCP materials according to thepresent invention.

According to a further aspect of the present invention there is provideda method of producing the intermediate compounds. The method maycomprise the use of the Bayliss-Hillman reaction (illustrated in Scheme1(b)), which has not hitherto been used in liquid crystal chemistry.

The method may also comprise reactions, other than the Bayliss-Hillmanreaction, to produce the intermediate compounds, as would more readilyoccur to one skilled in the art.

The liquid crystal polymer or copolymer may comprise intermediatecompounds, or monomers, of general structure (C): ##STR7## wherein X=CN,COR, CO₂ R, CHO, CF₃ ;

R=branched chain, straight chain or chiral alkyl and n may be 1 to 20.

The monomer unit my comprise any ore of the following examples 1.3a to1.3m. ##STR8##

The monomer units may be used to synthesise polymers or copolymers ofgeneral structure (D): ##STR9## wherein X=CN, COR, CO₂ R, CHO, CF₃ ;

R=branched chain, straight chain or chiral alkyl;

▭=any suitable mesogenic group;

n=1 to 20;

m=3 to 200;

The liquid crystal polymer or copolymer may comprise ##STR10##

The liquid crystal polymer or copolymer my comprise the generalstructure (E): ##STR11## wherein X=CN, COR, CO₂ R, CHO, CF₃ ;

R=branched chain, straight chain or chiral alkyl;

Z=single covalent bond, oxygen, sulphur, CO₂ or OCO;

n=1 to 20;

m=3 to 200;

For example, the liquid crystal polymer or copolymer may comprise##STR12##

The liquid crystal polymer or copolymer may comprise monomers of generalstructure (F): ##STR13## wherein X=CN, COR, CO₂ R, CHO, CF₃ ;

R=branched chain, straight chain or chiral alkyl;

▭=any suitable mesogenic group;

n=1 to 20;

For example, the liquid crystal polymer or copolymer may comprise:##STR14##

The liquid crystal polymer or copolymer may comprise monomers of generalstructure (G): ##STR15## wherein X=CN, COR, CO₂ R, CHO, CF₃ ;

R=branched chain, straight chain or chiral alkyl;

▭=any suitable mesogenic group;

Y=H, OH, F, OCH₃ ;

n=1 to 20;

The LCP material according to the invention may be used in any of theknown types of electro-optic device, for example in large area highresolution displays, piezo- and pyro-electric sensors and as componentsfor optical sensing. A device incorporating such a material is anotheraspect of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS SCHEME 1 Preparation ofCompounds of Structure (1.1)

An example of this procedure is the synthesis of6-(4-cyano-4'-oxybiphenyl)hexanol (1.1a). ##STR16##

A mixture of 4-cyano-4'-hydroxybiphenyl (0.102 mol),1-chloro-6-hydroxyhexane (0.122 mol) and potassium carbonate (0.130 mol)in molecular sieve-dried dimethylformamide (30 ml) was refluxed for 24h. The mixture was poured into water (2 L) and cooled to 4° C. Theresultant precipitated crystalline solid was removed by filtration andrecrystallised from aqueous acetone to give white crystals (yield=92%).

m/z 295 (M⁺), 195. υ_(max) (KCl disc) 3550-3000, 2950, 2880, 2230, 16051495, 1475, 1400, 1295, 1270, 1250, 1185 cm⁻¹. ¹ HNMR (CDCl₃) δ1.35-1.65 and 1.80-1.90 (m, 9H), 3.65 (t, 2H), 4.0 (t, 2H), 6.95-7.70(m, 8H). K 93 N 110° C. I.

A similar methodology was employed to synthesise the followingcompounds. ##STR17##

Preparation of Compounds of Structure (1.2)

This procedure (Ref 1) is exemplified by the synthesis of6-(4-cyano-4'-oxybiphenyl)hexanal (1.2a) ##STR18##

Dimethylsulphoxide (0.085 mol) dissolved in dichloromethane (18 ml) wasadded dropwise to oxalyl chloride (0.038 mol) dissolved indichloromethane (80 ml) and cooled to -30° C. A solution of6-(4-cyano-4'-oxybiphenyl) hexanol (1.1a) (0.034 mol) was added dropwiseover -10 min and the reaction mixture allowed to rise to -5° C. When thereaction was complete (t.l.c. monitoring), the reaction mixture wascooled to -30° C. and triethylamine (0.20 mol) added dropwise. Themixture was allowed to rise to room temperature and water (60 ml) wasadded. The organic phase was separated. The aqueous phase wasre-extracted with dichloromethane (50 ml), and combined dichloromethaneextracts plus the organic phase were washed with saturated brine (100ml), 1% v/v hydrochloric acid (100 ml), water (100 ml), aqueous sodiumcarbonate (5%) (100 ml), water 2×100 ml) and then finally dried overmagnesium sulfate. Removal of solvent in vacuo left a clear oil whichwas mixed with a solution of sodium metabisulphite (10 g) in water (25ml) and ethanol (5 ml), and shaken to form a bisulphite additioncompound. This was removed by filtration, washed with ethanol (2×100 ml)and then placed in saturated sodium carbonate solution (100 ml) andstirred for 45 min. Extraction with diethyl ether (3×100 ml) followed bydrying over magnesium sulphate and removal of solvent in vacuo left awhite waxy solid of one-spot purity by t.l.c.

Yield=78%. υ_(max) (KCl disc) 2950, 2870, 2230, 1725, 1605, 1495, 1290,1250, 1180, 825 cm⁻¹ m/z 293 (M⁺), 195. ¹ HNMR (CDCl₃) δ 1.50 (quin,2H), 1.70 (quin, 2H), 1.90 (quin, 2H), 2.50 (t, 2H), 3.95 (t, 2H),6.95-7.70 (m, 8H), 9.80 (s, 1H). K 78.5 (N 74.0)°C. I.

A similar procedure was used to synthesize the following compounds##STR19##

Preparation of Adducts (1.3)

This procedure (Ref 2) is exemplified by the synthesis of2-cyano-3-hydroxy-8-(4-cyano-4'-oxybiphenyl)oct-1-ene (1.3a). ##STR20##

A mixture of 6-(4-cyano-4'-oxybiphenyl)hexanal (1.2a) (0.0034 mol),acrylonitrile (0.0136 mol), 3-quinuclidinol (0.0011 mol) was stirred atroom temperature for 48 h. Excess acrylonitrile was removed in vacuo andthe resultant oily solid was dissolved in dichloromethane (50 ml) thenwashed with 2N hydrochloric acid (50 ml), water (2×100 ml), and finallydried over magnesium sulphate. Removal of solvent left a clear oil whichsolidified on cooling. Column chromatography using silica gel and (1)dichloromethane followed by (2) diethyl either as eluents gave a white,waxy solid of one-spot purity by t.l.c.

Yield=80%. m/z 346 (M⁺), 328, 195. υ_(max) (KCl disc) 3600-3200, 2970,2830, 2230, 1670, 1600, 1495, 1470, 1290, 1255, 1185 cm⁻¹. ¹ HNMR(CDCl₃) δ 1.30-1.85 (m, 8H), 1.95 (d, 1H), 3.95 (t, 2H), 4.20 (q, 2H),5.90 (s, 1H) 6.0 (s, 1H), 6.90 (d, 2H), 7.45 (d, 2E), 7.60 (q, 4H). K47.5° C. I.

A similar procedure was used to synthesis the following compounds.##STR21##

SCHEME 2 Preparation of Polymers (2.3) Preparation of Compounds ofStructure (2.1)

This procedure is exemplified by the synthesis of1-bromo-2-cyano-8-(4-cyano-4'-oxybiphenyl)oct-2-ene (2.1a). ##STR22##

Dimethyl sulphide (0.0104 mol) in dry dichloromethane (10 ml) was addeddropwise to a solution of N-Bromosuccinimide (0.0095 mol) in drydichloromethane (30 ml) cooled to 0° C. A white complex ofN-bromosuccinimide and dimethyl sulphide was precipitated. After themixture was stirred for 10 min at 0° C., a solution of2-cyano-3-hydroxy-8-(4-cyano-4'-oxybiphenyl)oct-1-ene (1.3e) (0.0087mol) was added dropwise. The mixture was then stirred for 24 h at roomtemperature during which time the solution cleared. The reaction mixturewas diluted with petroleum fraction (bp 40°-60° C.) (100 ml) and thenpoured into ice-water containing sodium chloride (200 ml). The organicphase was separated and washed with saturated brine (100 ml). Theaqueous phase was re-extracted with diethyl ether (2×50 ml) and combinedextracts plus the organic phase were dried over magnesium sulphate.Removal of solvent in vacuo left a clear oil. Column chromatographyusing silica gel and an equal mixture of petroleum fraction (bp 40°-60°C.) and dichloromethane gave a clear oil of one-spot purity by t.l.c.Recrystallisation from a 50:50 mixture of diethyl ether: petroleumfraction (b.p 40°-60° C.) gave colourless crystals. ##STR23##

Yield=67%. m/z 408, 410 (M⁺), 195. υ_(max) (KBr disc) 2940, 2860, 2220,1600, 1495, 1255, 1173, 823 cm⁻¹ 1 HNMR (CDCl₃) δ 1.55-1.30 (m, 4H),1.85 (quin, 2H) 2.45 (q, 2H), 4.0-4.10 (m, 4H), 6.50 (t, complex, 1H),7.0 (d, 2H), 7.55 (m, 2H), 7.65 (q, 4H).

A similar methodology was employed to synthesise the followingcompounds. ##STR24##

Preparation of Compound of Structure (2.3)

This procedure (Ref 3) is exemplified by the synthesis of polymer(2.3a). ##STR25##

1-Bromo-2-cyano-8-(4-cyano-4'-oxybiphenyl)oct-2-ene (2.1a) (0.0037 mol)was dissolved in sodium-dried tetrahydrofuran (2 ml) and the solutioncooled, under an atmosphere of dry nitrogen, to -70° C. Lithiumtriethylborohydride (1.0M solution in tetrahydrofuran) (0.0039 mol) wasadded dropwise and the temperature of the solution was allowed to riseto -20° C. when reaction commenced, as indicated by tlc. The monomerintermediate (2.2a) could not be isolated under these conditions andt.l.c. showed direct formation of polymer (2.3a). The reaction mixturewas added to ice-water (100 ml) and extracted with dichloromethane (2×50ml) and then dried over magnesium sulphate. Removal of the solvent lefta white solid. This was dissolved in dichloromethane (2 ml) and addeddropwise to petroleum fraction (bp 40°-60° C.) (50 ml) with vigorousstirring to form a fine suspension. Filtration followed by washing withpetroleum fraction (2×50 ml) and drying in air gave a fine white powder(0.65 g).

υ_(max) (KBr disc) 825, 1180, 1250, 1290, 1495, 1605, 2225, 2860, 2940cm⁻¹. ¹ HNMR (CDCl₃) δ 0.85 (q, 2H), 1.20-1.65 (m, 12H), 1.65-1.90(broad s, 2H), 3.80-4.05 (broad s, 2H), 6.80-7.00 (broad s, 2H),7.35-7.75 (complex, 6H). ¹³ CNMR (CDCl₃) 24.65, 25.95, 29.15, 67.95,110.20, 115.10, 119.20, 127.15, 128.40, 130.40, 131.45, 132.65, 145.20,159.75.

M_(w) =5360 M_(w) /M_(n) =1.6

M_(n) =3370

No. repeat units=10

K 58.8 N 104.0° C. I.

Preparation of Compound of Structure (2.2)

This procedure is exemplified by the synthesis of2-cyano-8-(4-cyano-4'-hexyloxybiphenyl)oct-1-ene (2.2). ##STR26##

1-Brom-2-cyano-8-(4-cyano-4'-oxybiphenyl)oct-2-ene (2.1a) (0.0052 mol)was dissolved in dry THF (10 ml) at room temperature. Sodium9-cyano-9-hydrido-9-borabicyclo 3.3.1!nonane (9-BBNCN) Ref 4! (1.0Msolution in the THF) (0.0052 mol) was added with stirring. After 20 mint.l.c. showed complete reaction. The solvent was removed in vacuo andthe product was purified by column Chromatography using silica gel withdiethyl ether:petroleum fraction (bp 40°-60° C.) (1:1) as eluent.

Yield=52%. ¹ HNMR (CDCl₃) δ 1.20-1.50 (m, 8H) 1.80 (quin, 2H), d4.0 (t,2H), 5.65 (s, 1/2H). 5.85 (s, 1/2H), 6.15 (t, 1/2H), 6.95 (d, 2H), 7.55(d, 2H), 7.65 (q, 4H). υ_(max) (Thin film) 2925, 2860, 2220, 1600, 1580,1520, 1490, 1465, 1595, 1290, 1265, 1200, 1180, 985, 820 cm⁻¹. m/z 346,344 (M⁺), 195.

This gave a mixture of 3 products in about equal quantities includingcompound (2.2).

SCHEME 3 Preparation of Compounds of Structure (3.2)

An example of this procedure is the synthesis of2-cyano-3-fluoro-8-(4-cyano-4'-oxybiphenyl)oct-1-ene (3.2a) ##STR27##

2-Cyano-3-hydroxy-8-(4-cyano-4'-oxybiphenyl)oct-1-ene (1.3a) (0.0058mol) was dissolved in dry dichloromethane (10 ml) and the solution wascooled to -78° C. Dimethylaminosulphur trifluoride (Methyl-DAST) (0.0069mol) was added slowly and the mixture stirred for 10 min before beingwarmed to room temperature. The resultant solution was then added toice-water (50 ml) and the product extracted into dichloromethane (2×25ml). The combined extracts were dried over magnesium sulphate andremoval of the solvent in vacuo gave a yellow oil which was thenpurified by column chromatography using silica gel with dichloromethaneas the eluent.

Yield=45%. m/z 348 (M⁺), 328 (M⁺ -20), 279, 195, 166, 149. υ_(max) (thinfilm) 2950, 2865, 2230, 1605, 1500, 1255, 1000, 825 cm⁻¹. ¹ HNMR (CDCl₃)δ 1.45-1.95 (m, 8H) 4.0 (t, 2H), 4.9-5.1 (m, 1H); 4.8 and 4.95 (2×s, F-Hdoublet) 6.1 (d, 2h), 6.95 (d, 2H), 7.55-7.70 (m, 6H).

A similar procedure was used to synthesise the following compound##STR28##

Preparation of Polymers of Structure (3.1 and (3.3)

An example of this procedure is the synthesis of polymer (3.1a).##STR29##

2-Cyano-3-hydroxy-8-(4-cyano-4'-oxybiphenyl)oct-1-ene (1.3a) (0.0044mol) and photoinitiator (Irgacure 184) (2 mol %) (0.09 mmol) weredissolved in dry dichloromethane (5 ml) and the resultant solutionevenly distributed over the inner surface of a 15 cm diameter Petridish. Evaporation of solvent left a thin film of monomer andphotoinitiator which was then exposed to sunlight or a sunlamp for 6hours. After this time, t.l.c. showed approximately 50% polymerformation and this proportion did not increase on further exposure tosunlight. The monomer/polymer mixture was suspended in dry methanol andcentrifuged for 30 min at 5×10³ rpm. The supernatant liquid containingmostly monomer was removed and the white precipitate dissolved indichloromethane (2 ml) and suspended in petroleum fraction (bp 40°-60°C.). The resultant fine precipitate was removed by filtration and driedin air to leave a fine white powder of the polymer (350 mg).

υ_(max) (KCl disc) 3700-3100, 2220, 1600, 1490, 1290, 1250, 1180, 820cm⁻¹. ¹ HNMR (CDCl₃) 1.35-1.65 (m, 10H), 1.80 (s, 2H), 3.95 (s, 2H),6.90 (s, 2H), 7.55 (s, 2H), 7.60 (s, 6H).

M_(w) =3798 M_(w) /M_(n) =1.24

M_(n) =3060

A number of other monomers have been polymerised using this procedure toproduce the following polymers.

    ______________________________________                                         ##STR30##                    (2.3a)                                           ##STR31##                    (3.1d)                                           ##STR32##                    (3.3a)                                           ##STR33##                    (3.3b)                                           ##STR34##                    (3.1b)                                           ##STR35##                    (3.1c)                                          ______________________________________                                        TRANSITION TEMPERATURES/°C.                                            Compound No                                                                   ______________________________________                                        (2.3a)K 40.0 N 64.0 I                                                         (3.3a)K 47.0 N 96.0 I                                                         (3.1b)K 25.5 S.sub.A 99.0 I                                                   (3.1d)K 42.0 N 73.0 I                                                         (3.3b)K 30.0 S.sub.A 96.0 I                                                   (3.1c)K 50.0 S.sub.A 71.0 I                                                   ______________________________________                                                                 Transition                                                                    Temperatures/°C.                              ______________________________________                                         ##STR36##               K 76.0 S.sub.A 113.0 I                                ##STR37##               K 38.O S.sub.A 66.0 I                                 ##STR38##               K 30.0 S.sub.A 49.0 I                                ______________________________________                                    

The following polymers were also synthesised and the subsequent dataobtained: ##STR39##

These compounds were made according to the method depicted in FIG. 1.The reagents used in FIG. 1 are:

(i) R(-)-2-octanol, diethylazodicarboxylate (DEAD), triphenylphosphine

(ii) magnesium, triethylborate

(iii) dihydropyran, p-toluenesulphonic acid

(iv) tetrakis (triphenylphosphine) palladium (o), sodium carbonate,toluene, followed by p-toluenesulphonic acid, dichloromethane

(v) 1-bromo-11-hydroxyhexane, potassium carbonate

(vi) potassium hydroxide, ethanol

(vii) DCC/DMAP

(viii) pyridinium dichromate, sodium acetate, dichloromethane

(ix) acrylonitrile, methyl acrylate or methyl vinyl ketone,3-quinuclidinol

(x) Irgacure 184, hv

where

DCC=dicyclohexylcarbodiimide

DMAP=4-N,N-dimethylaminopyridine

Irgacure 184 is a photoinitiator commercially available from Ciba Geigy.

The compounds described by the current invention are useful for a broadrange of applications.

Many of the compounds described by Formula A and mixtures includingcompounds of Formula A show liquid crystalline behaviour and are thususefully employed in liquid crystal devices. Examples of such devicesinclude optical and electro-optical devices, magneto-optical devices,and devices providing responses to stimuli such as temperature changesand total or partial pressure changes. The compounds of formula I mayalso be included in a mixture, Where the mixture comprises at least twocompounds. Typical mixtures include mixtures consisting of compounds ofFormula A, and also mixtures comprising at least one compound of FormulaA and at least one compound not of Formula A.

An example of the use of a compound of general structure A in a deviceembodying the present invention will now be described with reference toFIG. 2.

The liquid crystal device consists of two transparent plates, 1 and 2,in this case made from glass. These plates are coated on their internalface with transparent conducting electrodes 3 and 4. An alignment layer5,6 is introduced onto the internal faces of the cell so that a planarorientation of the molecules making up the liquid crystalline materialwill be approximately parallel or at a small angle to the glass plates 1and 2. For some types of display the alignment directions areorthogonal. The electrodes 3, 4 may be formed into row and columnelectrodes so that the intersections between each column and row form anx, y matrix of addressable elements or pixels. A spacer 7 eg ofpolymethyl methacrylate separates the glass plates 1 and 2 to a suitabledistance eg 2 microns. Liquid crystal material 8 is introduced betweenglass plates 1, 2 by filling the space in between them. The spacer 7 issealed with an adhesive 9 in a vacuum using an existing technique.Polarisers 10, 11 are arranged in front of and behind the cell. For somedevices, only one or even no polarisers are required.

In alternative embodiments the substrates with the aligning layers onthem are heated and sheared to induce alignment, alternatively thesubstrates with the aligning layers are thermally annealed above theglass transition temperature and below the liquid crystal to isotropicphase transition in combination with an applied field. Furtherembodiments may involve a combination of these aligning techniques. Withsome of these combinations an alignment layer may not be necessary.

The device may operate in a transmissive or reflective mode. In theformer, light passing through the device, eg from a tungsten bulb, isselectively transmitted or blocked to form the desired display. In thereflective mode a mirror (12) is placed behind the second polariser 11to reflect ambient light back through the cell and two polarisers. Bymaking the mirror partly reflecting the device may be operated both in atransmissive and reflective mode.

The alignment layers 5,6 have two functions one to align contactingliquid crystals molecules in a preferred direction and the other to givea tilt to these molecules--a so called surface tilt--of a few degreestypically around 4° or 5°. The alignment layers 5, 6 may be formed byplacing a few drops of the polyimide onto the cell wall and spinning thewall until a uniform thickness is obtained. The polyimide is then curedby heating to a predetermined temperature for a predetermined timefollowed by unidirectional rubbing with a roller coated with a nyloncloth.

In an alternative embodiment a dye material may be incorporated with thematerial of the device and a single polariser used.

Materials have been proposed for laser addressed applications in whichlaser beams are used to scan across the surface of the material or leavea written impression thereon. For various reasons, many of thesematerials have consisted of organic materials which are at leastpartially transparent in the visible region. The technique relies uponlocalised absorption of laser energy which causes localised heating andin turn alters the optical properties of the otherwise transparentmaterial in the region of contact with the laser beam. Thus as the beamtraverses the material, a written impression of its path is left behind.One of the most important of these applications is in laser addressedoptical storage devices, and in laser addressed protection displays inwhich light is directed through a cell containing the material and isprojected onto a screen. Such devices have been described by Khan Appl.Phys. Lett. Vol. 22, p 111, 1973; and by Harold and Steele inProceedings of Euro display 84, pages 29-31, September 1984, Paris,France, in which the material in the device was a smectic liquid crystalmaterial. Devices which use a liquid crystal material as the opticalstorage medium are an important class of such devices. The use ofsemiconductor lasers, especially Ga_(x) Al_(1-x) As lasers where x isfrom 0 to 1, and is preferably 1, has proven popular in the aboveapplications because they can provide laser energy at a range ofwavelengths in the near infra-red which cannot be seen and thus cannotinterfere with the visual display, and yet can provide a useful sourceof well-defined, intense heat energy. Gallium arsenide lasers providelaser light at wavelengths of about 850 nm, and are useful for the aboveapplications. With increasing Al content (x<1), the laser Wavelength maybe reduced down to about 750 nm. The storage density can be increased byusing a laser of shorter wavelength.

The compounds of the present invention may be suitable as opticalstorage media and may be combined with dyes for use in laser addressedsystems, for example in optical recording media.

The smectic and/or nematic properties of the materials described by thecurrent invention may be exploited.

For example the materials of the current invention may be used toproduce ferroelectric mixtures and devices.

The compounds of the present invention may also be used in pyroelectricdevices for example detectors, steering arrays and vidicon cameras.

FIG. 3 illustrates a simple pyroelectric detector in which the materialsof the current invention may be incorporated:

A pyroelectric detector consists of electrode plates 1,2 at least one ofwhich may be pixellated. In operation the detector is exposed toradiation R, for example infrared radiation, which is absorbed by theelectrode 1. This results in a rise in temperature which is transmittedto a layer of pyroelectric material 3 by conduction. The change intemperature results in a thermal expansion and a charge is generated.This change in charge is usually small when compared with the chargeouput due to the change in the spontaneous polarisation, Ps with achange in temperature; this constitutes the primary pyroelectric effect.A change in charge results in a change in potential difference betweenthe electrodes. The charge on each pixel may be read out and theresulting signal is used to modulate scanning circuits in, for example,a video monitor and for a visual image of the infrared scans.

REFERENCES

1. J Mancuso, D. Swern, Synthesis, 165;185 (1981).

2. A B Baylis and M E D Hillman, German Patent 2155113, 1972; Chem Abs77, 34174q (1972).

3. H M R Hoffman, J Rabe, J. Organ. Chem. 3849-3859 (1985).

4. R O Hutchins, J. Org. Chem., 42, 82-91 (1977).

We claim:
 1. A liquid crystal polymer or copolymer of structure (A):##STR40## wherein X is selected from the group consisting of CN, COR,CO₂ R, CHO and CF₃ ;R is selected from the group consisting of abranched chain, straight chain or chiral C₁₋₁₀ alkyl; ▭=a mesogenicgroup; Z is selected from the group consisting of a single covalentbond, oxygen, sulfur, CO₂ and OCO; n is 1 to 20; Y is selected from thegroup consisting of H, OH, F and OCH₃ ; m is 3 to 200; and P is selectedfrom the group consisting of a single covalent bond, oxygen and OCO. 2.A liquid crystal polymer or copolymer according to claim 1, wherein themesogenic group is defined from structure (B); ##STR41## wherein A, Band D are independently selected from the group consisting of: ##STR42##W₁ and W₂ are independently selected from the group consisting of asingle covalent bond, COO, OCO, CH₂ CH₂, CH₂ O, OCH₂ and 0;Q is selectedfrom the group consisting of CN, halogen, R, OR, COOR, OOCR, CF₃,lactate derivatives where R is a chiral/straight or branched chain C₁₋₁₀alkyl, and including where one or more CH₂ groups are optionallysubstituted by CH(CN), CH(CF₃), CH(Cl), CH(CH₃), where if more than oneCH₂ group is substituted then such substitution is not carried out onneighboring CH₂ groups.
 3. A liquid crystal polymer or copolymeraccording to claim 1, wherein Y is OH.
 4. A liquid crystal polymer orcopolymer according to claim 3 wherein the OH groups are used as a pointof attachment for cross-linking agents to form elastomers.
 5. A liquidcrystal polymer or copolymer according to claim 1 comprising monomers ofstructure (C): ##STR43##
 6. A liquid crystal polymer or copolymeraccording to claim 5 wherein X=CN, COR, CO₂ R, CHO, CF₃ ; R=branchedchain, straight chain or chiral alkyl and n=1 to
 20. 7. A liquid crystalpolymer or copolymer according to claim 5 wherein the monomer comprisesany one of compounds 1.3a to 1.3m: ##STR44##
 8. A liquid crystal polymeror copolymer according to claim 1, wherein the polymer or copolymercomprises the general structure (D): ##STR45## wherein X=CN, COR, CO₂ R,CHO or CF₃ ;R=branched chain, straight chain or chiral alkyl; ▭=anysuitable mesogenic group: n=1 to 20; m=3 to
 200. 9. A liquid crystalpolymer or copolymer according to claim 8 comprising ##STR46## m=3 to200; R=branched chain, straight chain or chiral alkyl.
 10. A liquidcrystal polymer or copolymer according to claim 3, wherein the polymercomprises the structure (F); ##STR47## wherein X=CN, COR, CO₂ R, CHO,CF₃ ;R=branched, straight or chiral alkyl; z=a single covalent bend,oxygen, sulphur, CO₂ or OCO; n=1 to 20; m=3 to
 200. 11. A liquid crystalpolymer or copolymer according to claim 10 comprising ##STR48##
 12. Aliquid crystal polymer or copolymer according to claim 10 comprising##STR49##
 13. A liquid crystal polymer or copolymer according to claim10 comprising ##STR50##
 14. A liquid crystal polymer or copolymeraccording to claim 1 comprising monomers of structure (G): ##STR51##wherein X=CN, COR, CO₂ R, CHO, CF₃ ;R=branched chain, straight chain orchiral alkyl; ▭=any suitable mesogenic group; and n=1 to
 20. 15. Aliquid crystal polymer or copolymer according to claim 14 comprising##STR52##
 16. A liquid crystal polymer or copolymer according to claim14 comprising ##STR53##
 17. A liquid crystal polymer or copolymeraccording to claim 1 comprising ##STR54## wherein X is CN, COCH₃ or CO₂CH₃ and n is 3-200.
 18. A liquid crystal material having opticallyactive properties and containing at least one liquid crystal polymer orcopolymer as claimed in claim
 1. 19. A liquid crystal material accordingto claim 18 wherein the material is a ferroelectric liquid crystalmaterial.
 20. A liquid crystal electro-optical display devicecharacterised in that it incorporates the material as claimed in claim18.
 21. A device comprising two spaced cell walls each bearing electrodestructures and treated on at least one facing surface with an alignmentlayer, a layer of a liquid crystal material enclosed between the cellwalls, characterised in that it incorporates the liquid crystal materialas claimed in claim
 18. 22. A pyroelectric device comprising two spacedelectrodes and a layer of a liquid crystal material enclosed between theelectrodes, characterised in that it incorporates the liquid crystalmaterial as claimed in claim
 18. 23. An optical recording mediumcomprising a recording layer which comprises one or more compounds ofclaim 1 and a dye material.
 24. A method of producing monomer units, notthemselves liquid crystalline, which polymerise to form liquid crystalpolymer materials as claimed in claim 1 using the Bayliss-Hillmanreaction.